Subchunk (/) Data Format

Byte Order

Subchunk data is stored in Little Endian byte order.

Parsing The Subchunk

NOTE: Bedrock subchunks are only saved to disk if they have changed since world generation.

Name	Size (in bytes)	Description
Version	1	The version of this subchunk.

If Version is 8 and 9

Name	Size (in bytes)	Description
Block Layer Count	1	The number of block layers this subchunk has.
Y-Index	1	The Subchunk Y Position (ranges from -4 to 19). (Only exists if Version is 9)

Now loop the next section Block Layer Count times until you have parsed all block layers.

Name	Size (in bits)	Description
Palette Type	1	The type of this palette (0 = Persistence, 1 = Runtime)
Bits Per Word	7	How many bits are used for each word.

If Palette Type is 0

Name	Size (in bytes)	Description
Layer Indices	ceil(4096[1] ÷ Blocks Per Word[2]) × 4	Stored as Blocks Per Word indices, with each unsigned 32-bit integer containing multiple indices, each occupying Bits Per Word bits. The indices are packed and stored in YZX order (Y increments first).
NBT Palette Compound Count	4	The number of NBT compounds to parse
NBT Palette Compounds	Variable	Multiple NBT compounds of individual blocks of the palette used in this subchunk

If Palette Type is 1 (Might not be used in practice)

Name	Size (in bytes)	Description
Layer Indices	ceil(4096[1] ÷ Blocks Per Word[2]) × 4	Stored as Blocks Per Word indices, with each unsigned 32-bit integer containing multiple indices, each occupying Bits Per Word bits. The indices are packed and stored in YZX order (Y increments first).
NBT Palette Compound Count	1 Signed VarInt (ZigZag)	The number of NBT compounds to parse
NBT Palette Compounds	Variable	Multiple NBT compounds of individual blocks of the palette used in this subchunk

---

If Version is 0 or between 2 and 7

NOTE: Second block layer for this version uses LegacyBlockExtraData (4) key instead, rather then being stored with the subchunk like in newer versions of the format.

Name	Size (in bytes)	Description
Block IDs	4096	8 bits per block. All the block ids for this chunk stored in YZX order (Y increments first)
Block Data	2048	4 bits per block. All the data values for the blocks in this chunk stored in YZX order (Y increments first)
Skylight	2048	4 bits per block. May be omitted. All the skylight data for this chunk stored in YZX order (Y increments first)
Blocklight	2048	4 bits per block. May be omitted. All the blocklight data for this chunk stored in YZX order (Y increments first)

Notes

1. In some languages you may need to write this as 4096.0 instead of just 4096 for the correct output.
2. Blocks Per Word = floor(32 ÷ Bits Per Word)